The present invention relates to a heat-shrinkable sheet, and more particularly to a heat-shrinkable sheet which is useful for covering and protecting fragile glass bottles or the like.
It is known to cover and protect fragile containers made of glass or the like for holding juice, a carbonated drink, a drink containing fruit juice, etc., with various types of heat-shrinkable foamed sheets (see U.S. Pat. No. 3,760,968 for example). Recently, there has been used a heat-shrinkable sheet excellent in printability and capable of preventing a container from scattering when the container is broken. The sheet has a laminated structure of a foamed sheet and a non-foamed film (see U.S. Pat. Nos. 3,979,000 and 4,463,861 for example).
Juice and drinks containing fruit juice require sterilization during bottling. Such sterilization is achieved according to a hot-fill method in which, after being sterilized by heat, the contents such as juice are sealed in a heated condition in a bottle and cooling water is then brought into contact with the bottle to cool the contents, or according to a cold-fill method in which the contents are sealed in a bottle in an ambient temperature and sterilized with heat by blowing boiling water, vapor or the like onto the bottle, and then contacting the bottle with cooling water to cool the contents.
The conventional heat-shrinkable foamed sheet above-mentioned having a smooth foamed surface is excellent in adiabatic ability (heat-insulating ability) and is used with the entire inner peripheral surface sticking, or adhering, to the bottle. Therefore, it is difficult for the boiling water or cooling water to penetrate between the bottle and the heatshrinkable foamed sheet and a long period of time is required for heating or cooling the contents after they are sealed in the bottle. This disadvantageously prevents efficient heat sterilization.
In view of the foregoing disadvantage, there is sometimes used a non-foamed heat-shrinkable film having a good thermal conductivity. However, such heat-shrinkable film lacks cushioning properties and does not provide sufficient protection of a bottle charged with contents. Such a film, therefore, is not suitable for a fragile container such as a glass bottle.
In order to adequately protect a fragile container such as a glass bottle and to achieve efficient heat sterilization, there has been proposed a heat-shrinkable sheet excellent both in cushioning properties and thermal conductivity. This heat-shrinkable sheet has a two-layer structure of a foamed layer and a non-foamed layer, the foamed layer having an internal structure (between the outermost surface of one side and the outermost surface of the other side) in which 1 to 50% of the cells is formed in a continuous cell structure (Unexamined Japanese Patent Publication 74837/1986).
In the heat-shrinkable sheet above-mentioned, as the percentage of the continuous cell structure of the foamed layer approaches 50%, the thermal conductivity is improved, but a large quantity of cooling water can penetrate the inside of the foamed layer and deteriorate the drying characteristics. If such a sheet as it is, is used on a container, there is a certain danger of mold growing to provide unhealthy conditions.
On the contrary, as the continuous cell rate approaches 1%, the thermal conductivity is deteriorated, thus preventing efficient heat sterilization.